Reflow bonding of borosilicate glass tubes to silicon substrates as fluidic interconnects

The subject of the thesis was the use of borosilicate glass tubes as an interface to waferlevel microfluidic devices. The tubes are compatible with the standard fluidic connectors and can be used as a package for the so-called MEMS-on-a-tube assembly. The connections are produced by the brief reflow of the glass tubes after they are placed on single crystal silicon wafers. At the bonding temperature, the glass at the interface with the silicon flows slowly over time enabling the voids at the interface to be filled, producing a permanent bond between the glass and the silicon substrate. The preparation and positioning of the samples in an oven and the formation of the bond at high temperature were explained and the selection of bonding parameters was discussed. The heterogeneous nucleation and growth of crystals in borosilicate glass during its heat treatment for the reflow bonding above its glass transition temperature were investigated, although the bulk of the borosilicate glass remained unaffected by crystallisation. The strength of the connections between borosilicate glass tubes and silicon substrates has been tested by pressurising it to failure. Failure occurred in the bonded materials rather than along the bond itself. The bond formed was hermetic. The only leakage was due to gas diffusion through the glass. The interface of borosilicate glass tube–silicon joins formed after the reflow bonding was studied by electron microscopy. Amorphous borosilicate glass has been found to be fully intact with silicon even at the atomic scale. The incorporation of in-plane electrical interconnects to reflow bonding has been studied. After joining the tubes to silicon substrates, the electrically conductive lines will allow probing into the volume confined by the tube. Therefore methods of fabrication of electrical interconnects, which would survive the bonding and not alter the quality of the bond interface, were investigated. Lastly, the examples of applications of borosilicate glass tubes as package or fluidic interconnects were given. The thesis was concluded with a detailed summary of the results and an outlook for the reflow bonding process.